title: Size-dependent subnanometer Pd cluster (Pd4, Pd6, and Pd17) water oxidation electrocatalysis.
abstract: Water oxidation is a key catalytic step for electrical fuel generation. Recently, significant progress has been made in synthesizing electrocatalytic materials with reduced overpotentials and increased turnover rates, both key parameters enabling commercial use in electrolysis or solar to fuels applications. The complexity of both the catalytic materials and the water oxidation reaction makes understanding the catalytic site critical to improving the process. Here we study water oxidation in alkaline conditions using size-selected clusters of Pd to probe the relationship between cluster size and the water oxidation reaction. We find that Pd4 shows no reaction, while Pd6 and Pd17 deposited clusters are among the most active (in terms of turnover rate per Pd atom) catalysts known. Theoretical calculations suggest that this striking difference may be a demonstration that bridging Pd-Pd sites (which are only present in three-dimensional clusters) are active for the oxygen evolution reaction in Pd6O6. The ability to experimentally synthesize size-specific clusters allows direct comparison to this theory. The support electrode for these investigations is ultrananocrystalline diamond (UNCD). This material is thin enough to be electrically conducting and is chemically/electrochemically very stable. Even under the harsh experimental conditions (basic, high potential) typically employed for water oxidation catalysts, UNCD demonstrates a very wide potential electrochemical working window and shows only minor evidence of reaction. The system (soft-landed Pd4, Pd6, or Pd17 clusters on a UNCD Si-coated electrode) shows stable electrochemical potentials over several cycles, and synchrotron studies of the electrodes show no evidence for evolution or dissolution of either the electrode material or the clusters.
authors.affiliation: Materials Science Division,Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA.
authors.forename: Gihan
authors.initials: G
authors.lastname: Kwon
authors.forename: Glen A
authors.initials: GA
authors.lastname: Ferguson
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Heard
authors.forename: Eric C
authors.initials: EC
authors.lastname: Tyo
authors.forename: Chunrong
authors.initials: C
authors.lastname: Yin
authors.forename: Janae
authors.initials: J
authors.lastname: DeBartolo
authors.forename: Sönke
authors.initials: S
authors.lastname: Seifert
authors.forename: Randall E
authors.initials: RE
authors.lastname: Winans
authors.forename: A Jeremy
authors.initials: AJ
authors.lastname: Kropf
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Greeley
authors.forename: Roy L
authors.initials: RL
authors.lastname: Johnston
authors.forename: Larry A
authors.initials: LA
authors.lastname: Curtiss
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Pellin
authors.forename: Stefan
authors.initials: S
authors.lastname: Vajda
mesh_terms: D002384:Catalysis; D004563:Electrochemistry / Q000295:instrumentation*; D004566:Electrodes; D004867:Equipment Design; D019544:Equipment Failure Analysis; D008422:Materials Testing; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure*; D010084:Oxidation-Reduction; D010165:Palladium / Q000737:chemistry*; D010316:Particle Size; D013499:Surface Properties; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D010165:Palladium